In recent years, rechargeable electric storage devices such as battery cells (lithium ion battery cells, nickel hydrogen battery cells, etc.) and capacitors (electric double layer capacitors, etc.) are employed as a power source, for example, for vehicles (automobiles, motorcycles, etc.) and various equipment (mobile terminals, laptop computers, etc.).
For example, a battery cell (rectangular battery cell) disclosed in Patent Literature 1 includes an electrode assembly (battery element), a case body (body case) that houses the electrode assembly, and a cover plate (cover) that closes an opening of the case body. The cover plate is provided with a step portion projecting toward the inside of the case body. Such a step portion allows the cover plate to be positioned at a specific position at the opening of the case body. This battery case is sealed by fitting the cover plate into the opening of the case body.
In Patent Literature 1, the step portion as an area excluding the circumferential edge portion of the cover plate planarly projects uniformly toward the inside of the case body. Therefore, a capacity equivalent to the volume of the step portion cannot be used as a capacity capable of housing the electrode assembly inside the case. That is, the capacity equivalent to the volume of the step portion is lost from the capacity inside the case. A reduction in the capacity inside the case causes a reduction in the space to house the electrode assembly that contributes to the electricity storage by the battery cell. [n1]Therefore the electric storage capacity per unit volume of the battery cell is reduced.
Such problems are not limited to batteries, and the same problems apply also to capacitors (such as electric double layer capacitors).